In electric motors used in automobiles, windings are housed in slots provided in an annular stator core (stator), and have a shape in which portions of the windings protrude from the stator into coil end portions that are outward in an axial direction.
In recent years, a demand has arisen to mount an electric motor without changing the volume of the engine room of the automobile. As such, there is a need to both miniaturize the electric motor, including the coil end portions, and also increase the output of the electric motor. If the length of the coil end portions is shortened so as to reduce the distance between adjacent conductors as much as possible to meet these needs, there will be an extremely high voltage difference between the conductors, which will lead to the occurrence of partial discharge and, as a result, insulation performance deteriorates.
An example of an electric motor in actual use is given to explain insulation performance deterioration caused by partial discharge. When an electric motor is in actual use, potential differences caused by drops in voltage occur between coils of the same phase that are electrically connected in series. Particularly, with stators in which coils of a plurality of phases have a star connection arrangement, the coils of the plurality of phases are connected by neutral points and current is input from the input terminal side of each phase. As such, depending on switching operations of the inverter, voltage may become concentrated in specific coils of each phase near each input terminal side due to the generation of steep surge voltage, which is higher than the voltage when driving with normal AC current, and the shared voltage of specific coils may increase. Particularly, in recent years, high speed switching has been investigated to reduce loss in inverters and, as a result, increased surge voltage and higher frequencies have become more common. Here, in cases where a specific coil and another coil other than the specific coil are connected within the same slot, and also in cases where the coil end portions of these coils are near each other, the potential difference between the adjacent coils increases due to the effects of the surge voltage. Moreover, in cases where the potential difference between the adjacent coils exceeds the Partial Discharge Inception Voltage (PDIV), partial discharge occurs between the coils, which leads to deterioration of the insulating portions between the coils. In other words, the insulation life of the coils may be shortened.
To solve this problem, providing insulating paper in the slots and partitioning the plurality of coils using the insulating paper has been considered. However, in such a method, the coil space factor within the slots declines and the length of the slots increases, which leads to an increase in the size of the electric motor. Additionally, if insulating paper is inserted into the slots, space referred to as “slot tip clearance”, which is provided at the inner end portion in the radial direction of the stator within the slots, may decrease, and copper vortex loss may increase. In this case, the loss of the electric motor may increase, resulting in a decrease in the fuel economy of hybrid vehicles and similar vehicles that use electric motors as motors for running or the like; loss may increase at portions of the coils that include portions disposed on the innermost peripheral portion of the slots, resulting in increases in coil temperature; or the like.
On the other hand, configuring all of the coils, including an entirety of an insulating layer provided around the periphery of the in-slot components, from an insulating layer having a dielectric constant that is lower than that of conventionally used insulating layers has also been considered. However, with such a method, the manufacturing cost of the stator may increase.
As such, in order to solve these problems, Patent Document 1 describes a technique to reduce the potential difference between coils of the same phase and coils of different phases and improve insulating performance. Specifically, Patent Document 1 describes a method of housing coils in slots such that input/output terminal side coils of the same phase and neutral point side coils where potential is high are adjacent within each phase slot, and input/output terminal side coils where potential is high and neutral point side coils where potential is low of different phases are adjacent outside the slots. In this method, of the plurality of partial coils forming the coils of each phase, inside-slot conductor portions of coils connected to the input/output terminal side where potential is high are housed in slots other than slots existing at circumferential end positions of each phase slot group.
Additionally, Patent Document 2 describes a technique to reduce voltage between coils. Specifically, Patent Document 2 describes housing a coil that has a first insulating layer with a first dielectric constant in a reference slot; and housing an input terminal side coil where potential is highest, which is provided with a layer with a second dielectric constant lower than the first dielectric constant, in a slot other than the reference slot.